The present invention relates to a tunable coupler device according to claim 1 and an optical filter incorporating said tunable coupler device according to claim 11.
More particularly, the present invention relates to a tunable coupler device designed for use in devices including but not limited to directional couplers, Mach-Zehnder interferometers, optical ring resonators, infinite impulse response filters, dispersion compensating devices, add-drop multiplexers, optical wavelength converters or optical modulators.
An optical signal is often split from one input port to two output ports for signal distribution or monitoring. This can be accomplished passively by using a directional coupler with two separate single-mode waveguides which are brought together for some interaction region. A gaussian-shaped single-mode wave propagating in a waveguide will have most of its energy residing in the core accompanied by an evanescent field tail propagating alongside the core within the cladding region. The evanescent tail of a single-mode wave, which is propagating along the interaction region in a first waveguide of a directional coupler, will therefore partially fall into the range of the second waveguide exciting an optical wave therein. In this way power is gradually coupled from the first to the second waveguide (see Mool C. Gupta, Handbook of PHOTONICS, CRC Press, Boca Raton 1997, pages 642-646).
According to Govind P. Agrawal, Fiber Optic Communication Systems, Wiley Series in microwave and optical engineering, New York 1992, chapter 6.2.1, pages 232-234, optical signals can be modulated by means of a Mach-Zehnder interferometer comprising two arms wherein the phase of optical carrier signals is shifted according to electrical binary data. As long as the phase of the optical carrier signals, which originate from the same source, is identical, the corresponding optical fields interfere constructively. An additional phase shift of adequate size introduced in one of the arms destroys the constructive nature of the interference of the optical carrier signals which are superpositioned on an output line of the ASK-modulator. The additional phase shift in the given example is introduced through voltage-induced index changes of the electro-optic materials (e.g. LiNbO3) used for said arms of the Mach-Zehnder interferometer.
In C. K. Madsen, G. Lenz, A. J. Bruce, M. A. Capuzzo and L. T. Gomez, Phase Engineering Applied to Integrated Optical Filters, IEEE Lasers and Electro-Optics Society, 12th annual meeting, San Francisco 1999, allpass filter rings and linear delay response architectures for dispersion compensations are described. A basic ring architecture consists of a tunable optical waveguide ring which is coupled to an optical waveguide through which optical signals are transferred. The thermo-optic effect is used to shift the phase of the signals within the ring. In order to obtain a desired filter response, it is critical to accurately fabricate the desired coupling ratio. To reduce the fabrication tolerances on the couplers and simultaneously to obtain a fully tunable allpass response, the basic ring architecture is preferably enhanced with a Mach-Zehnder interferometer (see FIG. 1). This enhanced ring structure, below called ring resonator, is briefly explained with reference to FIGS. 1 and 2.
FIG. 1 shows a prior art tunable balanced Mach-Zehnder interferometer with a first and a second waveguide 10, 11 aligned in parallel, with a first and a second directional coupler 31, 32, through which optical signals can be exchanged between said waveguides 10, 11, and with one thin-film heater 21 covering a part of the first waveguide 10 lying between the directional couplers 31, 32. An optical signal entering the first waveguide 10 at port A will partially be coupled in the first directional coupler 32 to the second waveguide 11. Between the directional couplers 31, 32 the phase of the remainder of the optical signal transferred in the first waveguide 10 will be shifted according to the thermal energy applied to the first waveguide 10 by means of the thin-film heater 21. The optical signal in the first waveguide 10 then interferes in the second directional coupler 32 with the optical signal of the second waveguide 11. Depending on the phase relationship between the optical signals, the signal intensity in the second waveguide 11 will be increased or reduced.
In case that the second waveguide 11 is formed as a ring and enhanced with a thin-film heater 22 for phase-shifting purposes, then the architecture shown in FIG. 1 corresponds to the tunable ring resonator shown in FIG. 2 respectively [3], FIG. 1 which may be used for dispersion compensation.
In order to obtain a desired shift of the phase of the optical signal in the first waveguide 10 relative to the phase of the optical signal in the second waveguide 11, thermal energy provided by the thin-film heater 21 is applied to the first waveguide 10 and not to the second waveguide 11. In the region of the thin-film heater 21 the waveguides 10, 11 are traditionally spaced apart at a distance which is sufficient to avoid a transfer of thermal energy from the thin-film heater 21 to the second waveguide 11.
Since the waveguides 10, 11 of the tunable ring resonator are kept apart from each other between the directional couplers 31, 32 over a relatively long distance, the architectures shown in FIGS. 1 and 2 are difficult to realize in small sizes as required for high frequency applications operating for example in the range of 25 GHz to 75 GHz.
It would therefore be desirable to create an improved tunable coupler device.
It would be desirable in particular to create a tunable coupler device which in conjunction with related circuitry can be fabricated at reduced cost and in high packing density.
More particularly it would be desirable to create a tunable coupler device which can easily be fabricated in planar waveguide technology.
It would further be desirable to create a tunable coupler device designed for use in devices including but not limited to directional couplers, Mach-Zehnder interferometers, optical ring resonators, infinite impulse response filters, dispersion compensating devices, add-drop multiplexers, optical wavelength converters or optical modulators.
The above and other objects of the present invention are achieved by a device according to claim 1 and a tunable optical filter according to claim 11.
The tunable coupler device is disposed on a substrate, comprising a first and a second waveguide for guiding optical signals, and comprising a heater element disposed adjacent the first waveguide in order to shift the phase of the optical signal in the first waveguide by means of the thermo-optic effect in response to a control voltage applied to the heater element. The heater element is disposed in an interaction region of the optical signals, such that, within the interaction region, a temperature gradient across the first and the second waveguide is generated in dependence on the applied control voltage. In order to increase the temperature gradient, the substrate may be designed to absorb thermal energy.
In order to further increase the temperature gradient, the heater element can be disposed adjacent the first waveguide and a heat sink element can be disposed adjacent the second waveguide such that thermal energy which passes from the heater element to the first and partially to the second waveguide is absorbed by the heat sink element.
In case that the second waveguide is formed as a closed loop or a ring, the heat sink element can be placed within said loop or ring, thereby reducing hindering the placement of further elements on the substrate.
Regions wherein optical signals are interacting and the region wherein the phase of the optical signals is shifted are therefore not separated in the tunable coupler device. Compared to prior art tunable couplers such as the balanced Mach-Zehnder interferometer shown in FIG. 1, which comprises two directional couplers enclosing a phase-shifting device, the herein described tunable coupler device comprises an overlapping phase-shifting and coupling section. The tunable coupler device can therefore be realized in significantly reduced size capable of operating in higher frequency regions.
In order to further reduce the size of the tunable coupler device for example implemented in optical filters the first waveguide can be bent along the second waveguide in the coupling region, so that the first or the second waveguide can be designed as ring with reduced diameter.
In a preferred embodiment of the invention, for example implemented in a tunable optical filter, an additional heater element is disposed adjacent the waveguide, which is forming a ring, in order to shift the phase of the optical signals circulating in the ring according to a control voltage applied to said additional heater element.
In order to improve coupling, the dimensions of the first and the second waveguide can be designed asymmetrically, the first waveguide being smaller than the second waveguide.
The heater elements preferably comprise a high resistive material such as Cr, Ni, Co or an alloy made thereof and the heat sink element and the leads connecting the heater elements to a voltage supply preferably comprise a low resistive material such as Al, Ag, Au or Cu.
The invention can be implemented advantageously in various optical circuits such as directional couplers, Mach-Zehnder interferometers, optical ring resonators, infinite impulse response filters, dispersion compensating devices, add-drop multiplexers, optical wavelength converters or optical modulators.